This invention relates to mold binder materials.
Molds for casting metal so far in practical use employ an inorganic binder such as sodium silicate, or employ phenol resin, furan resin and other organic binders. Any of these molds has a property to withstand the pressure or heat at the casting of molten metal, but the former has a poor collapsing ability of mold after the casting, and thus an operation to take out the casting is so difficult that number of steps of producing a casting is increased. The mold using an organic binder has no such problem.
As a process for hardening a mold, there are available a self hardening process by chemical reaction of binder components added to mold sands and a cold set process for hardening a mold by injecting a CO.sub.2 gas or an amine gas into a mold, after mold sands containing a binder are filled around a pattern. In the former process for hardening a mold, a chemical reaction starts at the same time when the binder is added to the mold sands, and thus, a time for effective utilization of mold sands, that is, the so-called working time, is restricted. The latter process includes the so-called CO.sub.2 process for hardening by injecting a CO.sub.2 gas into mold sands containing sodium silicate as a binder, a process for hardening by injecting an amine gas into mold sands containing a hydroxyl group-containing resin (for example, phenol resin) and a polyisocyanate as binders, and a process for hardening mold sands containing a combination of copolymer of acrylic acid ester and ammonium acrylate and sodium acrylate or phenolic resin, and calcium hydroxide by a CO.sub.2 gas. A longer working time of these mold sands can be assured than said chemical reaction of the binder components in the mold sands, and the mold can be immediately hardened by the injection of the gas, and thus the production efficiency of molds is better. However, since the CO.sub.2 process uses sodium silicate as a binder, the collapsing property of molds is poor, and the process using the amine gas has a problem in toxicity and smell of the amine gas. The process based on the combination of copolymers of acrylic acid ester and ammonium acrylate and sodium acrylate, and calcium hydroxide and a CO.sub.2 gas has the following problems: (1) an ammonia gas is evolved at the preparation of mold sands or shaping of molds, deteriorating the working atmosphere, and (2) a sufficient strength cannot be obtained right after the injection of CO.sub.2 gas.
Furthermore, the process using the phenol resin does not pollute the working atmosphere due to generation of ammonia gas, etc., but the hardening of the mold after the injection of the gas is conducted only by drying, and the strength of the mold is less increased.
The present inventors previously proposed a process for preparing a mold, based on a combination of polyvinyl alcohol and calcium hydroxide or calcium oxide, and a CO.sub.2 gas as art to improve the problems of CO.sub.2 process and other molds as said prior art, and the proposed process can advantageously improve the prior art considerably, but has a difficulty in uniform hardening to the inside of the mold, when a large mold is prepared.